You can, but remember with shapping you can only take material away, you can't add it and fish are typicaly wider then a thruster and have the wide point pushed forward.
with a 6'4" your only really talking about reshaping the nose, and chankging the fin position - there won't be a lot more you can do. changing just the nose won't alter the performance of the board too much so before you do this I would recomend trying the board with some wider fins as a twinny, this will give your board a much different feel and you may not even need to reshape the board to get what your looking for.
BTW here are some pics of my last home made board